7 1/4 to 8 3/4 rear end swap info needed

I think I went 31/4 but it's been many years and you have to measure and all that because you can go two deep or too shallow and have issues bottoming in the trans or the shaft falling out. It was supposed to be for a 727/ 7 1/4 combo though just like yours I think and why not use used shock plates, they are pretty stout so should be fine, mine were in the doner car and are still under mine, though I did later add a rear factory sway bar outa a 76 Valiant cop car, also a HUGE upgrade in handling if you want to go that route. Though those rear bars are made of unobtainium now, and were pretty hard to find then. I also believe if you do the right search you can find the factory length for a Dart 340 auto combo with the 83/4 and that is the length you'd need.

2852228 1967-69 3.25 47.81 727 8-3/4 340 383 Dart
2883693 1970 3.25 47.99 727 8-3/4 340 Dart